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Code No: R5220205 R5II B.Tech II Semester(R05) Supplementary Examinations, December 2010

ELECTRICAL MACHINES-II(Electrical & Electronic Engineering)

Time: 3 hours Max Marks: 80Answer any FIVE Questions

All Questions carry equal marks

1. (a) Explain the working principle and operation of Transformer.

(b) The voltage ratio of a single-phase, 50 Hz, transformer is 5000/500 V at no load. Calculate thenumber of turns in each winging if the maximum flux in the core is 7.82 mWb. [8+8]

2. (a) What is energy efficiency of transformer? Why energy efficiency of transformer is also importantspecification along with voltage regulation of transformer? Which types of transformers arespecified with energy efficiency?

(b) A 50 MVA transformer with tap changer has percentage impedance of 0.5 + j 7.0 . What

tapping (in %) must be used to maintain the rated voltage at the secondary on full load 0.8 pflagging & 40 MVA at 0.9 lagging? [8+8]

3. (a) Explain why open circuit test is conducted on transformer. Give related calculations.

(b) Calculate the efficiency of a 200/400 V, 4 kVA transformer at full load, pf. 0.8 lagging & half fullload, 0.75 leading with following test data:

OC test: 200 V, 0.8 A, 70 W (LV side)SC test: 20 V, 10 A, 60 W (HV side) [6+10]

4. (a) With neat phasor diagram, explain the voltage regulation of three-phase transformer.

(b) An ideal 3- step down transformer connected in delta/star delivers power to a balanced 3-load of 120 kVA at 0.8 pf. The input line voltage is 11 kV and the turns ratio of transformer

(phase to phase) is 10. Determine the line voltage line currents, phase voltages, phase currentson both primary & secondary sides. [8+8]

5. (a) A four pole/ eight pole multi speed 60 Hz, 10 HP, 240 V, 3-, IM operating with 4 poles runs at1750 RPM when fully loaded with its rated voltage & frequency. Determine:

i. slip speed

ii. % slip

iii. The synchronous speed if operating in the 8 pole mode and at 20 % rated frequency

(b) Obtain the direction of rotation of RMF, rotor, rotor MMF. Obtain the relation between them.[8+8]

6. The parameters of a 3- IM are r1 = 0.8 , x1 = 1.4 , r2

= 0.4 , x2

= 0.7 , Xo = 40 , Ro =160 . The supply voltages are Va = 150 at 00 , Vb = 250 at 900, Vc = 350 at 1100. If the slip is

0.04 find:

(a) air gap power for positive and negative sequence

(b) rotor copper loss for positive & negative sequence

(c) total mechanical power developed. [16]

7. With neat diagram explain the various tests to be conducted on 3- IM to plot the circle diagram.[16]

8. Explain all the modes of operation of Induction machine. Plot the neat characteristics. [16]

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